Rainproof Water Striders
taylorvich writes:
https://physics.aps.org/articles/v17/33
Water striders-lightweight insects that can walk on water-can spend their entire lives gliding across lakes and ponds. But it's not all plain sailing-when violent storms hit, raindrops dozens of times larger than the insects bombard the water's surface. Yet the insects come away unharmed. Now a team led by Andrew Dickerson of the University of Tennessee, Knoxville, has an explanation for how water striders withstand being hit by raindrops [1]. The results could help scientists understand how rain affects the transport of tiny particles, such as microplastics, through aquatic ecosystems.
Dickerson came up with the idea to study water striders on a rainy jog through the University of Central Florida's campus, where the ponds teem with the insect. To the insects, he thought, raindrops hitting the pond's surface must reverberate like the explosions of bombs. "What do these tiny bugs do when it rains?" he asked himself. The problem, it turned out, had not previously been explored.
For their experiments, the team used a "rain simulator," a device that mimics rainfall on a pond. The experimental drops were all about 4 millimeters in diameter, a diameter comparable with that of large natural raindrops. Falling from a few meters, the drops reached speeds of up to 6 meters per second by the time they hit the water striders resting on the artificial pond. The team filmed the impact of the drops on the water striders using a high-speed camera.
From the recorded movies, the team deduced that the impact of a drop on a strider triggers the following sequence of events. First, as the drop hits the surface a crater develops that pushes the strider underwater. The surface of the crater then rebounds, and a jet of liquid propels vertically upwards from the crater's center. Depending on the insect's position relative to the jet, the jet can launch the water strider into the air. But a strider can also get trapped within the boundaries of a secondary crater produced when the jet collapses back into the liquid. This crater then drags the trapped striders underwater. In their experiments, the researchers found that, in both cases, all water striders survived the drop impact.
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